Uintaite is a naturally occurring hydrocarbon mineral classified as an asphaltite. It is a natural product whose chemical and physical properties vary and depend strongly on the uintaite source. Uintaite has also been called "gilsonite" although this usage is incorrect; Gilsonite.RTM. is a registered trademark of American Gilsonite Co., Salt Lake City, Utah. American Gilsonite Co sells a variety of Gilsonite.RTM. "resins", sometimes identified by their softening points (ring and ball). For example, General Purpose (GP) Gilsonite.RTM. brand resin has a softening point of about 350.degree. F, and Gilsonite.RTM. HM has a softening point of about 380.degree. F, and Gilsonite.RTM. Select 300 and Select 325 which have softening points of about 300 and 325.degree. F, respectively. The softening points of these natural uintaites depend primarily on the source vein that is mined when the mineral is produced. Uintaite is described in the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed. Vol. 11, 198C, pp. 802-806, which is incorporated herein by reference. Typical uintaite used in drilling fluids is mined from an area around Bonanza, Utah and has a specific gravity of 1.05 with a softening point ranging from 370.degree. F to 400.degree. F, although a lower softening point (330.degree. F) material is sometimes used. It has a low acid value, a zero iodine number, and is soluble or partially soluble in aromatic and aliphatic hydrocarbons, respectively. It is generally processed and ground to where 99% passes through a 65-mesh Tyler screen with approximately 3% being retained on a 100-mesh screen and 16% on a 200-mesh screen, non-cumulative.
For many years uintaite and other asphaltic-type products have been used in water-based drilling fluids as additives assisting in borehole stabilization. These additives can minimize hole collapse in formations containing water-sensitive, sloughing shales. See, for example, SPE Paper 17203 (1988) to Davis and Tooman which is incorporated herein by reference in its entirety.
The causes of borehole instability are numerous. They can be mechanical, chemical, or physical in nature. Mechanical problems include borehole erosion by high annular velocities, adverse hydraulic stresses due to high annular pressures, hole collapse from high swab and surge pressures due to excessive wall cake, and stressed erosion due to drill string movement. Chemical alteration problems include hydration, dispersion, and disintegration of shales due to the interaction of clays with mud filtrate.
Physical instability problems include the spalling and rock bursts of shales due to in-situ rock stress and the difference between mud hydrostatic and formation pressures. Fracture and slippage along bedding planes of hard, brittle shales, and the collapse of fractured shales above deviated holes are also physical problems encountered while drilling troublesome shales.
Borehole instability problems are often referred to as sloughing, heaving, spalling, or overpressured shales, mud balls, mud rings, and many other descriptive names. There are many solutions to this problem. For example, additives have been used to inhibit or partially inhibit the swelling of clay. The adjustment of hydraulic conditions is another solution to reduce mechanical alteration. Knowing and controlling the pore pressure of the problem formations is used often.
Uintaite and asphalt-type materials have teen used for many years to stabilize sloughing shales and to reduce borehole erosion. Other benefits derived from these products include borehole lubrication and reduction in filtration. Numerous patents and publications disclose drilling mud additives containing uintaite.
Canadian Patent 972,141 to Sullivan discloses a well drilling fluid containing ground particles of uintaite (gilsonite) pre-coated with a non-ionic surfactant to make the particles water-wettable. Preferred surfactants are phosphate esters derived from the group consisting of polyoxyethylene ethers of organic compounds containing at least 6 carbon atoms and having a reactive hydrogen and condensed with at least 3 mols of ethylene oxide. A process for using this composition in well drilling operation is also disclosed.
U.S. Pat. No. 4,645,609 to Patel discloses mixtures of causticized lignite and sulfonated asphalts, including uintaite (gilsonite), as well-working fluids. U.S. Pat. No. 4,420,405 to McCrary discloses compositions and their methods of preparation comprising uintaite (gilsonite), lignite, tannin and a sulfonating compound, preferably sodium sulfite, as drilling mud additives.
U.S. Pat. No. 3,718,585 to Lummus et al. discloses an aqueous drilling fluid consisting of uintaite (gilsonite), a stabilizing agent and an alcohol. The stabilizing agent is a particular ethoxylated alkylphenol or a polypropylene glycol (Mw between 1700 and 3500) reacted with 8 to 13 wt.% ethylene oxide. The stabilizing agent is prehydrated to assist in dissolution and to prevent gum formation.
Although the above-described patents teach many improvements, a number of problems still exist when using uintaite-containing drilling fluid additives. Of particular importance is that no one additive is useful for the entire range of temperatures and pressures downhole.
Also, uintaite is not easily water wet with most surfactants. Thus, stable dispersions of uintaite are often difficult to achieve, particularly in the presence of salts, calcium, solids and other drilling fluid contaminants and/or in the presence of diesel oil. The uintaite must be readily dispersible and must remain water wet; otherwise it will coalesce and be separated from the drilling fluid, along with cuttings at the shale shaker or in the circulating pits.
Surfactants and emulsifiers are often used with uintaite drilling mud additives. Surfactant-coated uintaite powders are also known. However, the surfactant cf the surfactant-coated uintaite powders can lose its effectiveness over time, thereby causing stickiness or aggloxeration of the uintaite particles. Alternatively, the surfactant may evaporate leaving unwettable uintaite. Ideally the surfactant-coated uintaite should be storage stable for extended periods of time, without deterioration of performance.
Another serious problem with drilling mud additives is that they can be very sensitive to diesel fuel addition or contamination, resulting in kickout of the additive. Diesel kickout results in loss of the additive or the collecting screens.
Borehole stability tests of uintaite and other asphaltic-type additives have been conducted at ambient temperatures and pressures for many years. However, prediction of additive performance under downhole conditions has been unreliable. Borehole temperatures can vary from ambient up to 500.degree. F and pressures can vary from atmospheric up to 25,000 psi. Only recently have tests beer devised under simulated downhole conditions. The Downhole Simulation Cell (DSC) described in the SPE Paper 17202 by Simpson, Dearing, and Salisbury is one example of these new tests. Another is described in the paper of Davis & Tooman, SPE Paper 17203 which used a High Pressure/Hi9h Temperature Fluid Loss cell and Berea cores to determine the depth of intrusion of drilling mud additives into the Berea core. This paper compared commercially available uintaite and asphaltic additives under downhole conditions. Using these new test procedures, we have now developed additives with significantly improved performance.
It would be advantageous if a uintaite drilling mud additive composition could be provided that was water-wettable and could be rewet rapidly, that would be effective over a wide range of temperatures and pressures, that would have improved stability to diesel fuel contaminants in drilling wells, and that would be storage stable, maintaining rapid wettability over time.
It would be advantageous if a single uintaite drilling mud additive composition could be provided that was effective in preventing shale sloughing and minimizing borehole instability over a broad range of temperatures, such as those encountered from spud to total depth (TD), and did not require surfactant addition at the drill site.
One object of the present invention is to provide a composition that is useful in water-based drilling fluids to prevent shale sloughing.
Further objects of the invention will become evident to those skilled in the art by reading the following specification, including the examples and the claims.